The strings for sports equipment (e.g., tennis racquets) or musical instruments are usually coated with a thin layer at their outmost surface to improve their durability, spin, feeling, etc. Polyamide (nylon), polyester, and other polymers have been used to coat on strings. Nanocomposites, such as clay and carbon nanotube reinforced nylon 6 nanocomposites, having better physical properties than neat nylon 6, provide highly durable string coating materials with other functionalities. The reinforcing polymeric composites using nano-sized clay particles with high aspect ratio have been investigated since the 1980's (see U.S. Pat. No. 4,739,007). Strings are usually polymer materials with a multi-layer structure—core filament, wrapping filaments on the core filament, and coating. For the strings with multi-layer structures, coating materials are required to match the base materials and have good melt-flow properties (acceptable viscosity) at certain temperatures to enable them to penetrate into the gaps between the wrapping filaments. However, the viscosity of a nanocomposite is typically higher than the viscosity of neat nylon 6 at the same temperature. Thus, the nanocomposite may not easily penetrate into the gaps between the wrapping filaments. FIG. 1 shows an SEM image of a cross-section view of a nylon 6/clay nanocomposite coated on a wrapping filament, which shows that the nanocomposite material did not successfully fill in all of the gaps. The result is that many defects were left in the string resulting in an unacceptable durability of the strings. The gaps will result in chipping-off or unacceptable durability of coatings during high impact hitting of balls. Moreover, due to the creation of the gaps, these coatings also fail to sufficiently bond the filaments onto the core materials of the string. FIG. 2 is an SEM image showing the chipped materials from filaments and coatings after high impact tests on such strings coated in this manner.